[Gk.] The term applied in chemistry and mineralogy to those bodies which have assumed a regular geometrical form, in contradistinction to those substances which are amorphous. Although there are numerous varieties of crystalline form, they can all be reduced to six primary systems. These are distinguished from each other by the number and position of the axes, which are mathematical straight lines assumed to intersect each other in the centre of the crystal, and to connect the opposite faces of the crystal or its opposite corners. Each substance which crystallizes possesses a definite form, serving as one of the marks to distinguish it from other substances.